Switching regulators and linear regulators are well known types of voltage regulators for converting an unregulated voltage, such as a battery voltage, to a regulated DC voltage of a desired value. One type of switching regulator is a pulse width modulation (PWM) regulator that turns a switching transistor on and off at a certain frequency. In a conventional buck regulator topology, the power supply voltage is intermittently coupled to an inductor, and the inductor conducts a triangular current waveform to recharge an output filter capacitor. The charged filter capacitor provides a relatively constant voltage to the load. A feedback signal, which is typically the output voltage, determines when to shut off the switching transistor during each switching cycle. The switch on-time percentage is called the duty cycle, and this duty cycle is regulated so as to provide a substantially constant voltage at the output despite load current changes. There are many types of switching regulators.
A linear regulator, also referred to as a low dropout (LDO) regulator, controls the conductance of a transistor in series between the unregulated power supply and the output terminal of the regulator. The conductance of the transistor is controlled based upon the feedback voltage to keep the output voltage at the desired level.
Switching regulators are generally considered to be more efficient than linear regulators since the switching transistor is either on or off. When a transistor is fully on, such as in saturation or near the edge of saturation, the transistor is a highly efficient switch, and there is a minimum of wasted power through the switch. However, due to the pulsing of the current through the switch, a relatively large size filter circuit, consisting of an inductor and a capacitor, is needed so as to provide a low-ripple regulated voltage at the output. The inductor must be sized to not saturate at the highest rated load current for the switching regulator under worst case conditions. The size of the capacitor is based upon the frequency of the switching regulator and the allowable ripple. Accordingly, it is difficult to provide a very small switching regulator, including the filter circuitry, in a very small size while supplying a low-ripple regulated voltage.
A linear regulator, on the other hand, provides a very smooth output since the series transistor is always conducting. However, due to the large voltage differential across the transistor, power is wasted through the transistor, and substantial heat may be generated.
It is known to use a linear regulator at the output of a switching regulator to further smooth the output of the switching regulator for applications which require extremely steady regulated voltages. However, the resulting power supply is still relatively large due to the switching regulator inductor being sized so as not to saturate at the maximum load current under worst case conditions. The size of the inductor and capacitor dominate the overall size of the regulator.
What is needed is a smaller size regulator that supplies a very smooth regulated output with high efficiency.